The present invention relates to a charged particle beam drawing apparatus and a pattern forming method for drawing fine patterns by use of a charged particle beam. More particularly, the invention relates to a charged particle beam drawing apparatus and a pattern forming method suitable for fabricating semiconductor integrated circuits having extremely high degrees of integration.
The ever-expanding electronics industry is witnessing the rapid miniaturization of circuit patterns fabricated in semiconductor integrated circuits. Ever-finer patterns have been formed through widespread use of drawing methods based on a charged particle beam for higher resolution. One drawback of the typical charged particle beam drawing method is that the speed of drawing is low compared with methods for transferring circuit patterns through masks using a light beam. The disadvantage of reduced drawing speeds, however, has been gradually alleviated since the successful commercial use of a method for making comprehensive drawing of fine pattern areas about five to 10 micrometers square each.
Where circuit patterns are formed by use of a charged particle beam, attempts to draw ever-finer patterns can result in inaccurately formed line widths and/or gaps which are flanked by large and fine geometries or which are located at portions whose dimensions change abruptly. This phenomenon has turned out to be a major problem in the formation of refined patterns. The problem, called the proximity effect, must be resolved before the fine pattern forming method using a charged particle beam can work effectively. The cause of the phenomenon is well known: an emitted charged particle beam passes through a sensitive material (called the resist hereunder) to enter a semiconductor substrate from which part of the charged particles backward-scattered inside return to the resist for further exposure. The returning charged particles called backward-scattered particles exert the same effect brought about when a wide, fuzzy drawing pattern is exposed to feeble light. Additional exposure caused by the backward-scattered particles leads to overexposure of regions where the density of pattern drawing is high. The result is the phenomenon of the incorrectly formed line widths and gaps mentioned above.
As a way of minimizing the proximity effect, the inventors of this invention has proposed an exposure area density method in Japanese patent Laid-open No. Hei 3-225816. The proposed method involves dividing beforehand the entire drawing area into a plurality of smaller regions and calculating the exposure area density in each of the divided smaller regions. The exposure time is then made shorter for the smaller regions in which the exposure area density is relatively high, and made longer for the smaller regions in which the exposure area density is relatively low.
Another way of minimizing the proximity effect is what is known as the auxiliary exposure method discussed illustratively by G. Owen and P. Rissman in xe2x80x9cProximity effect correction for electron beam lithography by equalization of background dosexe2x80x9d (J. Appl. Phys., Vol. 54, No. 6, pp. 3573-3581; June 1983). According to this method, the exposed and unexposed portions of a drawing pattern are reversed to form an inverted pattern. The inverted pattern is reexposed to a defocused charged particle beam in a supplementary fashion to make uniform reexposure levels of backward scattering over the entire reexposed area (the procedure is called supplementary exposure), whereby the proximity effect is minimized. Japanese patent Laid-open No. Hei 5-160010 discloses a technique for determining the intensity of supplementary exposure for each of the divided smaller regions in proportion to its pattern area.
The exposure area density method above works on the principle of making drawing while correcting the exposure time for each of smaller regions using calculated values of region-by-region exposure area densities that vary depending on the position of drawing. Where each fine pattern of a certain size is transferred as a whole before being drawn, each pattern transferred in a lump constitutes a drawn geometry of a single smaller region, so that the exposure time of the smaller region in question is controlled at a constant level. In that case, in any one of the proximate smaller regions around the smaller region in question, there can be a discrepancy between the actual exposure time and an ideal exposure time. This phenomenon is conspicuous if patterns each transferred in a lump have large dimensions, and is particularly pronounced at portions where the exposure area density varies abruptly. As a result, some of the finished fine patterns fail to comply with the required dimensions.
According to the supplementary exposure method above, the effects of backward scattering are smoothed out by supplementary exposure. While minimizing incorrectly finished dimensions, this method requires two things: huge quantities of drawing data representing the inverted pattern must be prepared before exposure of that pattern, and a new set of equipment is needed to defocus the charged particle beam for irradiation. To meet these requirements takes time and thereby lowers throughput, i.e., reduces efficiency in dealing with individual wafers one by one. Furthermore, the supplementary exposure method involves having supplementary exposure carried out a number of times even if the exposure area density is relatively low. This often leads to the problem of the beam apparatus body tube being inordinately charged up.
It is therefore an object of the present invention to overcome the drawbacks and deficiencies of the conventional exposure area density method and supplementary exposure method and to provide a charged particle beam drawing apparatus and a pattern forming method capable of drawing fine patterns while minimizing the proximity effect.
In carrying out the invention and according to one aspect thereof, there is provided a pattern forming method comprising the steps of performing supplementary exposure by irradiating a drawing area on a specimen with a charged particle beam, and performing main exposure by irradiating with the charged particle beam a region made up of the drawing pattern inside the drawing area on the specimen. The supplementary exposure step includes the steps of: dividing the drawing area into a plurality of smaller regions of an equal area each while calculating an area value of each smaller region exposed to the charged particle beam; correcting the area value of each smaller region by use of a weighted sum of the area values calculated for proximate smaller regions surrounding the smaller region in question; generating supplementary exposure geometries for the plurality of smaller regions; and computing a dose of exposure for the generated supplementary exposure geometries by referring to the corrected area values.
Other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings.